Oxygen content variation and cation doping dependence of(La)1.4(Sr1−yCay)1.6Mn

Abstract

The results of the synthesis and characterization of the optimally doped ͑La͒ 1.4 ͑Sr 1−y Ca y ͒ 1.6 Mn 2 O 7±␦ solid solution with y = 0, 0.25, and 0.5 are reported. By progressively replacing the Sr with the smaller Ca, while keeping fixed the hole concentration due to the divalent dopant, the "size effect" of the cation itself on the structural, transport, and magnetic properties of the bilayered manganite has been analyzed. Two different annealing treatments of the solid solution, in pure oxygen and in pure argon, allowed us to also study the effect of the oxygen content variation. Structure and electronic properties of the samples have been investigated by means of x-ray powder diffraction and x-ray absorption spectroscopy measurements. Magnetoresistivity and static magnetization measurements have been carried out to complete the samples characterization. Oxygen annealing of the solid solution, which showed a limit for y ϳ 0.5, induces an increase of the Mn average valence state and a transition of the crystal structure from tetragonal to orthorhombic while the argon annealing induces an oxygen understoichiometry and, in turn, a reduction of the Mn average valence state. Along with the Ca substitution, the Jahn-Teller distortion of the MnO 6 octahedra is reduced. This has been directly connected to a general enhancement of the transport properties induced by the Ca doping. For the same cation composition, oxygen overstoichiometry leads to higher metal-insulator transition temperatures and lower resistivity values. Curie temperatures ͑T C ͒ are reduced by increasing the Ca doping. The lower T C for all the annealed samples with respect to the as-prepared ones are connected to the strong influence on the magnetic interactions of the point defects due to the ␦-variation.

Electrical properties of fluorine-intercalated layered manganite: La1.4Sr1.6Mn2O7F2

“…All the structural parameters are tabulated in Table 1. Both the lattice constants of a and c of the parent material are closely matched with earlier reports [22][23][24]. The a axis of La [16,17].…”

Pressure induced phase separation in optimally doped bilayer manganites

“…Susceptibility measurement on the La 1.4 Sr 1.6 Mn 2 O 7 compound revealed a clear paramagnetic to FM transition at about 120 K. The La 1.4 Sr 1.6 Mn 2 O 7 sample was synthesized as described in Ref. 5. HP x-ray diffraction data ͑ = 0.41 Å͒ were collected on the ID09 beamline at ESRF ͑Grenoble͒ with a diamond anvil cell ͑DAC͒ loaded with He and with low fluoresence IA 350 m culet diamonds.…”

Absence of Long-Range Magnetic Order in the La1.4Sr0.8Ca0.8Mn2O7 Bilayered Manganite

“…The main effect of the thermal treatment in pure oxygen is the oxidation of the Mn ions. The average Mn-valence state of the sample changes from about 3.27 (argon) to 3.38 (oxygen) as we previously determined by means of X-ray absorption spectroscopy at the Mn−K edge …”

“…Pellets were prepared from well-mixed powders and sintered at 1223 K for at least 4 days, during which time they were re-ground and re-pelletized at least twice. The as-prepared polycrystalline samples were annealed at 1123 K in pure argon ( p (O 2 ) = 1 × 10 -6 atm) and pure oxygen ( p (O 2 ) = 1 atm) to obtain a first sample with an average Mn valence state of about 3.27 and a second sample with an average Mn valence state of about 3.38 …”

“…In particular we were interested in looking at the role of mixed cation doping on the A-site of the structure. Up to now only one group and our earlier investigations , considered the (La) 1.4 (Sr 1 - y Ca y ) 1.6 Mn 2 O 7 ± δ solid solution. However, in ref only resistivity measurements are given.…”